length of his day, and of his night. The fun's disk will appear ninety times lefs to an inhabitant of Saturn than it does to us; but notwithstanding the fun appears fo fmall to the inhabitants of the regions of Jupiter and Saturn, the light that he will afford them is much more than would be at first fuppofed; and calculations have been made, from which it is inferred, that the fun will afford 500 times as much light to Saturn, as the full moon to us; and 1600 times as much to Jupiter. To eyes like our's, unaffifted by inftruments, Jupiter and the Georgium Sidus would be the only planets feen from Saturn, to whom Jupiter would fometimes be a morning, fometimes an evening flar.

One of the firft difcoveries of the telescope, when brought to a tolerable degree of perfection, was, that Saturn did not appear like other planets. Galileo, in 1610, fuppofed it compofed of 3 ftars, or globes, a larger in the middle, and a fmaller on each fide; and he continued his obfervations till the two leffer ftars difappeared, and this planet looked like the others. Further obfervation fhewed, that what Galileo took for two ftars, were parts of a ring. This fingular and curious appendage to the planet Saturn, is a thin, broad, opake ring, encompaffing the body of the planet, without touching it, like the horizon of an artificial globe, appearing double when viewed through a good telefcope. The fpace between the ring and the globe of Saturn, is fuppofed to be rather more than the breadth of the ring; the plane of the ring is inclined to the plane of the ecliptic, in an angle of 30 degrees, and is about 21,000 miles in breadth. It puts on different appearances to us, fometimes being feen quite open, at others only as a line upon the equator. It is probable that it at times cafts a fhadow over vaft regions of Saturn's body. This

ring fufpended round the body of the planet, and keeping it's place without any connection with the body, is quite different from all other planetary phenomena with which we are acquainted. But this is rendered ftill more surprising by the difcoveries of Dr. Herschel, who finds that the planet Saturn has two concentric rings, of unequal dimenflons and breadth fituated in one plane, which is probably not much inclined to the equator of the planet. Thefe rings are at a confiderable diftance from each other, the fmalleft being much less in diameter at the outfide, than the largest is at the infide the two rings are entirely detached from each other, fo as plainly to permit the open heavens to be feen through the vacancy between them. Of the nature of this ring, various and uncertain were the conjectures of the firft obfervers; though not more perplexed, than those of the lateft. Of it's ufe to the inhabitants of Saturn, we are as ignorant as of it's nature.

Saturn is not only furnished with this beautiful ring, but it has alfo feven attendant moons.

OF THE GEORGIUM SIDUS. H

From the time of Huygens and Caffini, to the discovery of the Georgium Sidus by Dr. Herfchel, though the intervening fpace was long, though the number of aftronomers was increased, though affiduity in obferving was affifted by accuracy and perfection in the inftruments of obfervation, yet no new difcovery was made in the heavens, the boundaries of our fyftem were not enlarged. The inquisitive mind naturally inquires, why, when the number of thofe that cul

tivated the science was increased, when the science itself was so much improved, in practical discoveries it was fo deficient? A fmall knowledge of the human mind will anfwer the queftion, and obviate the difficulty. The mind of man has a natural propenfity to indolence; the ardour of it's purfuits, when they are unconnected with felfish views, are foon abated, fmall difficulties difcourage, little inconveniences fatigue it, and reafon foon finds excufes to juftify, and even applaud this weakness. In the prefent inftance, the unmanageable length of the telescopes that were in ufe, and the continual expofure to the cold air of the night, were the difficulties the aftronomer had to encounter with; and he foon perfuaded himself, that the fame effects would be produced by fhorter telescopes, with equal magnifying power; herein was his mistake, and hence the reason why fo few discoveries have been made fince the time of Caffini. A fimilar inftance of the retrogradation of fcience occurs in the hiftory of the microscope, as I have fhewn in my effays on that inftrument.

The Georgium Sidus was difcovered by Dr. Herschel, in the year 1781; for this discovery he obtained, from the Royal Society, the honorary recompence of Sir Godfrey Copley's medal. He named the planet in honour of his Majefty King George III. the patron of fcience, who has taken Dr. Herschel under his patronage, and granted him an annual falary. By this munificence he has given fcope to a very uncommon genius, and enabled him to profecute his favourite ftudies with. unremitted ardour.

In fo recent a difcovery of a planet fo diftant, many particulars cannot be expected. It's year is fuppofed to be more than 80 years; it's diameter

34,299 miles; distance from the fun about 1,832 millions of miles; the inclination of it's orbit 43 degrees 35 feconds; it's diameter, compared to that of the earth, as 431,769 to 1; in bulk it is 8,049,256 times as large as the earth. It's light is of a blueifh white colour, and it's brilliancy between that of the moon and Venus.

Though the Georgium Sidus was not known as a planet till the time of Dr. Herschel, yet there are many reasons to fuppofe it had been feen before, but had then been confidered as a fixed star. Dr. Herschel's attention was first engaged by the fteadiness of it's light; this induced him to apply higher magnifying powers to his telescope, which increased the diameter of it: in two days he observed that it's place was changed; he then concluded it was a comet; but in a little time he, with others, determined that it was a planet, from it's vicinity to the ecliptic, the direction of it's motion, being stationary in the time, and in fuch circumftances as correfpond with fimilar appearances in other planets.

With a telescope, which magnifies about 300 times, it appears to have a very well-defined vifible difk; but with inftruments of a fmaller power it can hardly be diftinguished from a fixed ftar between the fixth and feventh magnitude. When the moon is abfent, it may also be seen by the naked eye.

Dr. Herschel has fince difcovered, that it is attended by two fatellites: a difcovery which gave him confiderable pleasure, as the little fecondary planets feemed to give a dignity to the primary one, and raise it into a more confpicuous fituation among the great bodies of our folar system. As the diftances of the planets, when marked

in

D3

in miles, are a burden to the memory, aftronomers often express their mean distances in a fhorter way, by fuppofing the diftance of the earth from the fun to be divided into ten parts. Mercury may then be estimated at four of such parts from the fun, Venus at seven, the earth at ten, Mars at fifteen, Jupiter at fifty-two fuch parts, Saturn at ninety-five, and the Georgium Sidus 190 parts.

TABLES

J